CA2264433A1 - Methods and apparatus for inter-frequency handoff in a wireless communication system - Google Patents
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- 238000004891 communication Methods 0.000 title claims abstract description 23
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
- H04W36/302—Reselection being triggered by specific parameters by measured or perceived connection quality data due to low signal strength
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/14—Reselecting a network or an air interface
Abstract
Inter-frequency handoffs in a CDMA or other wireless communication system are controlled using a noise-limited coverage trigger metric which is able to distinguish between same-frequency cell boundaries and other-frequency cell boundaries in the system. The trigger metric may be generated as a function of an average signal-to-noise measure for pilot signals received at a mobile station of the system and a linear sum of the signal-to-noise measures. The signal-to-noise measures may be generated in the mobile station and included in messages transmitted from the mobile station to one or more base stations of the system. The trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call. The trigger metric may alternatively be based on a measure of mobile receive power alone. Other aspects of the invention reduce unnecessary searching for a new frequency and decrease the likelihood of "ping-ponging" from a current frequency to a new frequency by providing additional checks in the handoff process. For example, receive power and pilot signal-to-noise measures may be generated at a mobile station for both current and new frequencies. The mobile station then continues to operate at the current frequency as long as certain predetermined threshold conditions based on the measures are satisfied.
Description
10152025CA 02264433 1999-03-03METHODS AND APPARATUS FOR INTER-FREQUENCY HANDOFFIN A WIRELESS COMMUNICATION SYSTEM The present invention relates generally to wireless communication systems andmore particularly to techniques for performing interâfrequency handoffs in wireless codedivision multiple access (CDMA) systems and other types of wireless systems. An important issue in IS-95 CDMA cellular and Personal Communications Service(PCS) wireless communication systems involves frequency handoffs between adjacent cellsor antenna sectors. Each of the cells in such a system generally includes a base station, andthe base station associated with a given cell may include an omnidirectional antenna or amultiple-sector directional antenna for communicating with mobile stations such asportable telephone handsets. As a mobile station moves throughout the system, itsposition relative to the system base stations changes, such that an ongoing call or othercommunication may need to be handed off from one base station to another, or from oneantenna sector to another. Adjacent base stations and antenna sectors are typicallyconï¬gured to utilize different communication frequencies in order to minimize effects suchas co-channel interference. Handoffs from one cell or sector to another may thereforeinvolve changing the communication channel frequency from a current frequency to a newfrequency. Such handoffs are generally referred to as inter-frequency or other-frequencyhandoffs.A number of techniques have been proposed for improving the efficiency andsuccess rate of interâfrequency handoffs in IS-95 CDMA systems. These techniques utilizemessages such as Extended Handoff Direction Messages (EHDNIS), Other FrequencyNeighbor List Messages (OFNLMS), Other Frequency Neighbor List Response Messages(OFNLRMS) and Other Frequency Report Messages (OFRMS) to implement inter-frequency handoffs. Although proposed techniques based on these messages can reducethe number of call drops during a frequency transition, these techniques still suffer from1015202530CA 02264433 1999-03-032a number of drawbacks. For example, the OFRM message is presently conï¬gured to allowa mobile station to report a signalâto-noise measure which is both âinterference limitedâin that it will typically decrease as the mobile moves across same-frequency cellboundaries, and ânoise limitedâ in that it will also decrease as the mobile moves acrossother-frequency cell boundaries. For a measure which is interference limited, interferencedue to signals generated by other cells is greater than the noise level, while for a measurewhich is noise limited, the noise level is greater than the interference due to signalsgenerated by other cells. A same-frequency cell boundary may be deï¬ned by a set ofpoints at which the strength of a pilot signal from one cell exceeds that of a pilot signalfrom an adjacent cell, where both the pilot signals are at the same frequency. An other-frequency cell boundary may be defined as a set of points at which a signal from one cellat a designated frequency exceeds the strength of a signal from an adjacent cell at anotherfrequency by a specified number of decibels. An OFRM message which utilizesconventional signalâto-noise measures cannot be used to distinguish same-frequency cellboundaries from otherâfrequency cell boundaries, and therefore does not provide anoptimal trigger for inter-frequency handoffs.In addition, the above-noted message-based techniques will often involve a basestation commanding a mobile station to perform a periodic search for a new frequency assoon as the mobile station enters into a transition area near the edge of a new cell orsector. However, this periodic search tends to degrade voice quality of an ongoing call,while also reducing the speed of the search for new potential base stations at the currentfrequency. Moreover, in many practical applications, this periodic search for a newfrequency can be unnecessary if the mobile is operating under certain types of radiofrequency (RF) conditions. Yet another significant problem with the above-notedtechniques is that the techniques may increase the likelihood of âping~pongingâ or rapidswitching between the new frequency and the current frequency. More particularly, it maybe possible in some areas of the system that both the new frequency and the currentfrequency will have good RF coverage, which could lead to pingâponging if, for example,the mobile station reports the received power and signalâto-noise measure for only the newfrequency.1015202530CA 02264433 1999-03-03 The invention provides methods and apparatus for improving inter-frequencyhandoffs in CDMA and other types of wireless communication systems. In accordancewith a ï¬rst aspect of the invention, a noiseâlimited coverage trigger is provided which maybe used to distinguish between sameâfrequency cell boundaries, which are generallyinterference limited, and other-frequency cell boundaries, which are generally noise limited.The coverage trigger is used to control inter-frequency handoffs, and can be implementedusing signal-to-noise measurements performed in a mobile station. In an illustrativeembodiment, the coverage trigger may be generated as the difference between the averagetransmit signal-to-noise measure for all significant pilots and the linear sum of significantpilot signal-to-noise measures reported in a Power Measurement Report Message(PMRM) or Pilot Strength Measurement Message (PSMM) transmitted from the mobilestation.Alternative embodiments may utilize the mobile receive power alone as a triggermetric. For example, a measure of mobile receive power incorporated into a PSMM canbe used to trigger a handoff to another frequency using a âdatabaseâ approach. In thisapproach, when the mobile receive power in a given cell becomes small and the mobilesees primarily border cell pilots, a particular pilot is selected from a list of neighbor pilotsstored in a database for that cell, and the mobile is instructed to perform a âblindâ handoffto the selected pilot at the new frequency. In this manner,\'a mobile can be instructed toperform a handoff to a new frequency without taking any pilot Ec/Io measurements at thenew frequency. The mobile receive power can also be used in the mobile to filter periodicreports. For example, the mobile may only make PSMM reports when the mobile receivepower drops below a threshold, which may be specified by the current cell site.Other aspects of the invention provide alternative techniques for controlling inter-frequency handoffs in a wireless communication system. These techniques cansubstantially eliminate unnecessary periodic searches in a frequency transition area, whilealso reducing the likelihood of pingâponging between a current frequency and a newfrequency. In an illustrative embodiment, the invention involves adding additional ï¬eldparameters to an Extended Handoff Direction Message (EHDM) of an IS-95 CDMA10152025CA 02264433 1999-03-034system The additional ï¬eld parameters include one or more thresholds which ensure thata mobile station initiates a search for a new frequency, for example, only if its receivedpower at the current frequency is lower than a certain threshold, or if a sum of signal-to-noise values for its active pilot signals at the current frequency is less than a certainthreshold. The mobile can also be conï¬gured to operate such that if the received powerat the new frequency does not exceed the received power at the current frequency by adesignated hysteresis amount, then a search is not performed in the new frequency. Theinvention thus provides additional checking criteria for use in interâfrequency handoffs toensure that unnecessary tuning and search in the new frequency is avoided.Further improvements can be provided in interâfrequency handoffs in otherillustrative embodiments of the invention. For example, a mobile station in an area inwhich a transition is to be made from a current frequency to a new frequency is conï¬guredto report received power and signal-to-noise values for both the new frequency and thecurrent frequency. If these values indicate that the mobile station is operating underacceptable RF conditions at the current frequency, there is no need to perform an inter-frequency handoff. This aspect of the invention can substantially reduce the likelihood ofpingâponging between the current and new frequencies, as compared to the conventionalinterâfrequency handoff techniques noted above.. . i h .FIG. 1 shows an exemplary code division multiple access (CDMA) wirelesscommunication system in which interâfrequency handoffs in accordance with the inventionmay be implemented.FIG. 2 is a block diagram showing an exemplary mobile station which can performinterâfrequency handoffs in accordance with the invention.FIGS. 3A and 3B show a flow diagram illustrating a frequency handoff processwhich may be implemented in the mobile station of FIG. 2 in accordance with one aspectof the invention.1015202530CA 02264433 1999-03-03Ilgtailgd |!gsg;;'jp];iQn of the inventionThe present invention will be illustrated below in conjunction with an exemplaryISâ95 code division multiple access (CDMA) wireless communication system. It shouldbe understood, however, that the invention is not limited to use with any particular typeof communication system, but is instead more generally applicable to any wireless systemin which it is desirable to provide improved performance for frequency handoffs. Forexample, although the techniques are illustrated with reference to the ISâ95 CDMAcellular and personal communications service (PCS) systems, it will be apparent to thoseskilled in the art that the techniques are also applicable to other CDMA systems, as wellas to other types of wideband and narrowband wireless systems. The term âprimary basestationâ as used herein refers generally to a base station communicating directly with agiven mobile station, such as the base station handling an on-going call for the mobilestation. The term âcurrent frequencyâ refers generally to the channel frequency used bya mobile station for an ongoing call or other communication. The term ânew frequencyârefers to a potential channel frequency to which an ongoing call or other communicationmay be handed off as the mobile station moves within the wireless system.FIG. 1 shows an exemplary cellular or personal communication services (PCS)system 10. The system 10 is configured in accordance with TIA/BIA/ISâ95A, âMobileStation ~ Base Station Compatibility Standard for Dual~Mode Wideband Spread SpectrumCellular System,â June 1996, and ANSI J-STD-008, âPersonal Station â Base StationCompatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA)Personal Communication Systems,â both of which are incorporated by reference herein.The system 10 includes a mobile station (MS) 12 and a number of base stations BS 1, BS2,BS3 and BS4. The base station BS1 of FIG. 1 represents a primary base station,communicating with mobile station 12 via the path designated by solid two-way arrow 24,while the surrounding base stations BS2, BS3 and BS4 may serve as secondary basestations which can detect signals transmitted via the paths indicated by dashed one-wayarrows 26. primary base station BS1 communicates with the mobile station 12 usingCDMA techniques described in the above-cited standards documents. As the mobilestation 12 moves within the system 10, handoffs occur, such that base stations other than1O15202530CA 02264433 1999-03-036BS1 become primary base stations for communicating with the mobile station 12. Thesystem 10 in this illustrative embodiment also includes first and second mobile switchingcenters (MSCS) 14-1 and 14-2. A given MSC typically connects several BSs with a publicswitched telephone network (PSTN) 16. For example, MSC 14-1 connects base stationsBS1 and BS2 with the PSTN 16 and MSC 14-2 connects base stations BS3 and BS4 withthe PSTN 16. The system 10 also includes a memory 18 having a number of registersincluding a home location register (I-ILR) 20 and a visitor location register (VLR) 22. TheI-ILR 20 and VLR 22 store user data and billing information for each mobile station 12 ofthe system 10.FIG. 2 shows a more detailed view of the mobile station 12. The mobile station12 includes an antenna 32 which receives signals from and transmits signals to basestations of the system 10. A receive signal is directed by a diplexer filter 33 to an input ofa receiver 34 which may implement conventional downconversion, demodulation, digital~toâanalog conversion and other processing of the receive signal. A transmitter 35performs complementary operations such as analogâtoâdigital conversion, modulation andupconversion to generate a transmit signal which is directed via diplexer 33 to the antenna32 for transmission. A processor 36 is coupled to both the receiver 34 and the transmitter35. The processor 36 operates in conjunction with a memory 38 to control thecommunication functions of the mobile station 12. For example, data or other informationin messages which are part of a receive signal may be supplied to processor 36 such thatprocessor 36 can implement the process steps to be described in conjunction with FIGS.3A and 3B below. The processor 36 may also perform receive signal power and signalâtoânoise measurements, and generate messages which are incorporated into the transmitsignal for transmission to one or more base stations.A first aspect of the invention relates to a noise-limited coverage trigger which,unlike conventional interâfrequency handoff triggers, may be used to distinguish betweenâinterference-limitedâ same-frequency cell boundaries, and ânoise-limitedâ otherâfrequencycell boundaries. The coverage trigger is used to control inter-frequency handoffs, and canbe implemented using signa1âtoânoise measurements performed in a mobile station. As willbe described in greater detail below, an exemplary coverage trigger in accordance with the10152025CA 02264433 1999-03-037invention may be generated as the difference between the average transmit signalâto-noisemeasure for all significant pilots and the linear sum of significant pilot signalâto-noisemeasures reported in otherwise conventional Power Measurement Report Message(PMRM) or Pilot Strength Measurement Message (PSMM) transmitted from the mobilestation. For example, an exemplary trigger metric Tn in accordance with the inventionmay be defined as:Tn = Fe - Eswhere Fe is the average transmit Ec/Io value in dB for all significant pilots, and Es is thelinear sum of all significant pilot Ec/Io values reported in the PMRM or PSMM messagesfrom the mobile station. The term âsigniï¬cant pilotsâ refers generally to pilots which areno less than X dB below the largest pilot as measured at the mobile, where X may beapproximately six or another suitable value. An Ec/Io value for a given pilot is a measureof the ratio of signal energy to interference plus noise at the pilot frequency. A typicalhandoff threshold for Tn might be between about 3 dB and 5 dB.This portion of the description will utilize the notational convention that valuesexpressed in logarithmic quantities (i.e., dB) begin with an upper case character, whilevalues expressed in linear quantities begin with a lower case character. For example, theaverage fraction fe of transmit power in the pilots may beexpressed as fe = 10F ââ 1°. Thefraction fe is given by:ï¬=z@memwhere e is a sorted (1, n) vector containing all Ec/Io values for the significant pilots, f isa (1, n) vector of all transmit Ec/Io values corresponding to the elements of e, n is thenumber of significant pilots as seen from the mobile station, and e./f is an operation whichdivides each element of e by the corresponding element inf. If e and f have a length of 1then fe = f . The vector f of transmit Ec/Io values may be expressed as:10152025CA 02264433 1999-03-038f = gpâ -/ (gpz +ga2 +gsâ + X(gv2))where gp is a (1, 11) Vector of pilot gain settings for each cell site corresponding to theelements of e, ga and gs are (1, n) vectors of page and sync channels in digital gain units(DGUs), gv is an (m, 71) matrix of voice charmel gains, and 20 applied to an (m, 11) matrixsums the colurrms to yield a (1, n) vector. The above-noted linear sum of Ec/Io values isthen given by:es = Z(e).The computation of the trigger metric Tn can be repeated periodically, such asabout every two to ï¬ve seconds, when the mobile station is in a frequency transition area.The computed metric is used to determine whether an interâfrequency handoff should takeplace. For example, a trigger metric having a Value which exceeds a threshold may beused to indicate the need for a particular inter-frequency handoff. Unlike conventionalhandoff triggers, the aboveâdescribed trigger is able to distinguish sameâfrequency cellboundaries from otherâfrequency cell boundaries, and is therefore particularly well suitedfor use in controlling inter-frequency handoffs.Alternative embodiments of the invention may utilize a handoff trigger which isbased on mobile receive power alone. For example, a measure of mobile receive powerincorporated into a PSMM can be used to trigger a handoff to another frequency using anapproach based on neighbor information stored in a database. A handoff which utilizesthis approach may be initiated when the mobile receive power in a given cell becomessmall and the mobile sees primarily border cell pilots. A border cell may be characterizedas a cell which is missing some neighbors on the same frequency. A particular pilot isselected from a list of neighbor pilots stored in a database for the given cell, and the mobileis instructed to perform a âblindâ handoff to the selected pilot at the new frequency. Inthis manner, a mobile can be instructed to perform a handoff to a new frequency withouttaking any pilot Ec/Io measurements at the new frequency. The mobile receive power canalso be used in the mobile to ï¬lter periodic reports. For example, the mobile may only1015202530CA 02264433 1999-03-039make PSMM reports when the mobile receive power drops below a threshold, which maybe speciï¬ed by the current cell site.FIGS. 3A and 3B show a flow chart which illustrates an inter-frequency handoffprocess in accordance with another aspect of the invention, as carried out by mobilestation 12 of system 10 in a frequency transition area. The frequency transition area maybe, for example, an area in which the mobile station 12 is approaching the vicinity of oneor more base stations while maintaining an ongoing call or other communication with aprimary base station. Step 40 of FIG. 3A indicates that the mobile station 12 iscommunicating with a base station over a CDMA traffic channel. The mobile station 12in step 42 receives an Other Frequency Neighbor List Message (OFNLM) from a basestation, in step 44 sends out an Other Frequency Neighbor List Response Message(OFNLRM) in response to the OFNLM message, and in step 46 receives an ExtendedHandoff Direction Message (EHDM). The OFNLM, OFNLRM and EHDM are describedin detail in the above-cited ISâ95 standard document, and provide information regardinginter-frequency handoff in the CDMA system 10.In steps 48 and 50, the mobile station 12 checks whether the received power at thecurrent frequency utilized by the CDMA trafï¬c channel is greater than a thresholdMIN_RX_PWR_CURR, and whether the sum of Ec/Io values for the active pilots isgreater than a threshold MlN_SUM_ECIO_CURR. Ifeither of these checks is not passed,the process moves to step 52 in which the mobile station tunes to a new frequency in orderto explore a potential handoff opportunity. If both of the checks of steps 48 and 50 arepassed, then the operation of tuning to the new frequency in step 52 is not performed sincethe mobile station is under acceptable RF conditions at the current frequency. AcceptableRF conditions may be defined as conditions such that the mobile received Ec/Io value iswithin Y dB of the transmit Ec/Io value, where Y is typically 3 or less. The checks ofsteps 48 and 50 help to eliminate the unnecessary tuning and searching operations typicallyassociated with conventional inter-frequency handoff techniques.If either of the checks of steps 48 and 50 are not passed, such that the mobilestation tunes to a new frequency in step 52 to explore a potential handoff opportunity, themobile station then in step 54 checks whether the received power at the new frequency is1015202530CA 02264433 1999-03-0310greater than a threshold MIN_RX_PWR_NEWF. The mobile station in step 56 alsochecks whether the difference between the received power at the new frequency and thereceived power the a thresholdHYSTERESIS__RX_PWR. This check serves to further reduce the likelihood thatat current frequency is greater thanunnecessary tuning and searching operations will be carried out, while also reducing ping-ponging between the current and new frequencies. If either of the checks in steps 54 and56 is not passed, then the mobile âstation in step 55 tunes back to the current frequency,and sends an Other Frequency Report Message (OFRM) which reports the followingmeasurements: (1) the received power at the current frequency (RX_PWR__CURR); (2)the received power at the new frequency (RX_PWR_NEWF); and (3) the Ec/Io values ofall active pilots measured at the new frequency. The Ec/Io value for a given pilot is ameasure of the signal energy of that pilot to the noise plus interference at the pilotfrequency. Other types of signal-to-noise measures may also be used.If both of the checks in steps 54 and 56 are passed, then the mobile stationperforms a search for a new active set of frequencies and/or a neighbor list for the newï¬equency. In step 58, a determination is made as to whether the new active set is empty.If the new active set is empty, the mobile station searches in step 59 for other flaggedpilots in the OFNLM message received in step 42. Additional details regarding this typeof searching in an IS-95 CDMA system can be found in the aboveâcited IS-95 standarddocument and, for example, E. Tiedemann and T. Chen, âInterâFrequency Hard HandoffImprovements (Rev. 2),â Qualcomm contribution to TR45.5, TR45.5.3.1/97.03.20.02,March 20, 1997, and P. Jain et al., âProposed IS-95-B Text for InterâFrequency HardHandoff Improvements,â Qualcomm contribution to TR45.5/97.03.20.03, March 17-21,1997, which are incorporated by reference herein.If the new active set is not empty, the mobile station in step 60 checks whether thesum of Ec/Io values for all of the active pilots in the new active set is greater than athreshold M]N_SUM__ECIO__NEWF. If the check of step 60 is not passed, the processgoes to step 59. to search for other ï¬agged pilots in the OFNLM message. If the check ofstep 60 is passed, the mobile station in step 62 uses the new active set while also searchingfor other ï¬agged pilots in the OFNLM message. A determination is made in step 64 as to1015202530CA 02264433 1999-03-0311whether the mobile station has received a âgoodâ frame within a designated waiting periodMAX_WAIT. A good frame is generally one in which substantially all of the bits in theframe have been received correctly. If a good frame is received within the MAX_WAITperiod, the mobile station in step 66 completes the inter-frequency handoff by sending anHCM, and continues the ongoing call on the new frequency.If a good frame is not received within the MAX_WAIT period in step 64, or afterthe search for other pilots in step 59 is commenced, the process moves to step 70 of FIG.3B. Ifthe search for other pilots is determined in step 70 to be the first search after receiptof the EHDM in the mobile station, the mobile station in step 71 tunes back to the currentfrequency, and sends an OFRM which reports the following measurements: (1) thereceived power at the current frequency (RX_PWR__CURR); (2) the received power at thenew frequency (RX_PWR_NEWF); (3) the Ec/Io values of all active pilots at the currentfrequency; and (4) the Ec/Io values of all active pilots at the new frequency. Providing thisinformation in the OFRM ensures that the base station will have better control on thehandoff trigger. Step 71 is also performed if an âalways reportâ condition is determinedto exist in step 72, or if the sum of the Ec/Io values of the ï¬agged pilots in the OFNLMmessage is greater than a threshold MlN_SUM_ECIO_NEWF in step 74. In the âalwaysreportâ condition, a ï¬ag will be set which tells the mobile to send an OFRM regardless ofwhether or not the sum of the new frequency pilots is above a threshold. If none of theconditions in steps 70, 72 and 74 are met, the mobile station tunes back to the currentfrequency in step 76, and then continues the call at the current frequency, as shown in step78.As noted previously, if both of the checks of steps 48 and 50 are passed, then thetuning to the new frequency in step 52 is not performed since the mobile station is underacceptable RF conditions at the current frequency. The process instead moves to step 80of FIG. 3B. If step 80 indicates that the process is on its first search for other ï¬aggedpilots after receipt of the EHDM in the mobile station, the mobile station in step 82 sendsan OFRM which reports the following measurements: (1) the received power at theâcurrent frequency (RX_PWR_CURR); and (2) the Ec/Io values of all active pilots at thecurrent frequency.1015202530CA 02264433 1999-03-0312A determination is then made in step 84 as to whether periodic search is to beperformed. Performance of periodic search may be speciï¬ed by a ï¬ag which is set toindicate that the mobile will search the new frequency periodically without furtherinstructions to search. If periodic search is not being performed, the mobile stationcontinues the call at thepcurrent frequency as shown in step 86. If periodic search is beingperformed, the mobile station continues the call at the current frequency as shown in step78, as long as step 88 indicates that the current time modulo the designated search periodis not equal to zero. When the time modulo the search period is equal to zero in step 88,the process returns to step 48 of FIG. 3A. Step 84 and its subsequent operations are alsoperformed if either of the checks in steps 54 and 56 is not passed, after the mobile stationin step 55 tunes back to the current frequency and sends the above-noted OFRM reportingRX_PWR_CURR, RX_PWR_NEWF and the Ec/Io values of all active pilots measuredat the new frequency.The inter~frequency handoff process described above in conjunction with FIGS.3A and 3B may be implemented by altering otherwise conventional ISâ95 EHDM andOFRM to include a number of additional fields for providing the above-noted thresholds.For EHDM may be the thresholdMIN__RX_PWR_CURR used in step 48, the threshold MIN_SUM_ECIO__CURR used instep 50, and the threshold HYSTERESIS_RX_PWR used in step 56. The OFRM couldbe modified such that it includes ï¬elds for measurements such as RX_PWR_CURR,RX_PWR_NEWF, the Ec/Io values of all active pilots at the current frequency, and theexample, the modiï¬ed to includeEc/Io values of all active pilots at the new frequency, as shown in step 71. The inter-frequency handoff process of FIGS. 3A and 3B can thus be implemented using simplemodiï¬cations to message formats in conjunction with appropriate programming ofsoftware, ï¬rmware or hardware in processor 36 and memory 38 of the mobile station 12.The foregoing description of the invention is intended to be illustrative only. Forexample, it should be noted that the EHDM, OFRM and other messages utilized in theabove description are exemplary only, and the interâfrequency handoff techniques of theinvention may be incorporated using other types of messages or signaling and with othertypes of wireless systems. In addition, the Ec/Io measurements reported by a mobileCA 02264433 1999-03-0313station may be replaced with other types of signalât'o-noise measurements or mobilereceived power measurements. These and numerous other alternative embodiments withinthe scope of the following claims will be readily apparent to those skilled in the art.
Claims (30)
1. A method of controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the method comprising the steps of:
generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and utilizing the trigger metric to control a handoff from a current frequency to a new frequency in an ongoing call.
generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and utilizing the trigger metric to control a handoff from a current frequency to a new frequency in an ongoing call.
2. The method of claim 1 wherein the generating step includes generating the trigger metric as the difference between an average transmit signal-to-noise measure for the plurality of pilot signals and a linear sum of pilot signal-to-noise measures for the plurality of pilot signals.
3. The method of claim 1 wherein the signal-to-noise measures are generated in the mobile station and included in messages transmitted from the mobile station.
4. The method of claim 3 wherein the system is an IS-95 CDMA system and the messages transmitted from the mobile station include at least one of a Power Measurement Report Message (PMRM) and a Pilot Strength Measurement Message (PSMM).
5. The method of claim 1 wherein the trigger metric is configured so as to distinguish same-frequency cell boundaries from other-frequency cell boundaries.
6. An apparatus for use in controlling frequency handoffs in a wireless communication system in which a mobile station communicates with one or more base stations, the apparatus comprising:
a processor for generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and a memory for at least temporarily storing a representation of the trigger metric, such that the trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call in the system.
a processor for generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and a memory for at least temporarily storing a representation of the trigger metric, such that the trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call in the system.
7. The apparatus of claim 6 wherein the trigger metric is generated as the difference between an average transmit signal-to-noise measure for the plurality of pilot signals and a linear sum of pilot signal-to-noise measures for the plurality of pilot signals.
8. The apparatus of claim 6 wherein the signal-to-noise measures are generated in the mobile station and included in messages transmitted from the mobile station to the one or more base stations.
9. The apparatus of claim 8 wherein the system is an IS-95 CDMA system and the messages transmitted from the mobile station include at least one of a Power Measurement Report Message (PMRM) and a Pilot Strength Measurement Message (PSMM).
10. The apparatus of claim 6 wherein the trigger metric is configured so as to distinguish same-frequency cell boundaries from other-frequency cell boundaries.
11. A method of controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the method comprising the steps of:
generating measures of receive power and pilot signal-to-noise for both a current frequency and a new frequency; and utilizing at least a subset of the measures to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency.
generating measures of receive power and pilot signal-to-noise for both a current frequency and a new frequency; and utilizing at least a subset of the measures to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency.
12. The method of claim 11 wherein the generating step includes generating a measure of receive power at the current frequency, a measure of receive power at the new frequency, a sum of pilot signal-to-noise measures at the current frequency, and a sum of pilot signal-to-noise measures at the new frequency.
13. The method of claim 12 wherein the system is a CDMA system and the receive power measures and the sums of pilot signal-to-noise measures are generated in the mobile station and transmitted in an Other Frequency Report Message (OFRM) to the one or more base stations.
14. The method of claim 11 further including the step of continuing to operate at the current frequency if the receive power at the current frequency is greater than a threshold.
15. The method of claim 11 further including the step of continuing to operate at the current frequency if a sum of signal-to-noise measures at the at the current frequency is greater than a threshold.
16. The method of claim 11 further including the step of continuing to operate at the current frequency if a difference between the receive power at the new frequency and the receive power at the current frequency is greater than a threshold.
17. The method of claim 11 further including the step of sending one or more threshold values to the mobile station in a message transmitted from one of the base stations, wherein the threshold values are used to determine if the ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency.
18. An apparatus for controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the apparatus comprising:
a processor which is operative (i) to obtain measures of receive power and pilot signal-to-noise for both a current frequency and a new frequency, and (ii) to utilizing at least a subset of the measures to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency; and a memory for at least temporarily storing one or more thresholds for use by the processor in determining if the ongoing call should continue at the current frequency or be handed off to the new frequency.
a processor which is operative (i) to obtain measures of receive power and pilot signal-to-noise for both a current frequency and a new frequency, and (ii) to utilizing at least a subset of the measures to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency; and a memory for at least temporarily storing one or more thresholds for use by the processor in determining if the ongoing call should continue at the current frequency or be handed off to the new frequency.
19. The apparatus of claim 18 wherein the measures obtained by the processor include a measure of receive power at the current frequency, a measure of receive power at the new frequency, a sum of pilot signal-to-noise measures at the current frequency, and a sum of pilot signal-to-noise measures at the new frequency.
20. The apparatus of claim 19 wherein the system is a CDMA system and the receive power measures and the sums of pilot signal-to-noise measures are generated in the mobile station and transmitted in an Other Frequency Report Message (OFRM) to the one or more base stations.
21. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if the receive power at the current frequency is greater than a threshold.
22. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if a sum of signal-to-noise measures at the at the current frequency is greater than a threshold.
23. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if a difference between the receive power at the new frequency and the receive power at the current frequency is greater than a threshold.
24. The apparatus of claim 18 wherein the one or more threshold values are supplied to the mobile station in a message transmitted from one of the base stations.
25. A method of controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the method comprising the steps of:
generating a trigger metric as a function of a measure of receive power at the mobile station; and utilizing the trigger metric to control a handoff from a current frequency to a new frequency in an ongoing call, wherein the handoff is performed without utilizing any signal-to-noise measures for pilot signals at the new frequency.
generating a trigger metric as a function of a measure of receive power at the mobile station; and utilizing the trigger metric to control a handoff from a current frequency to a new frequency in an ongoing call, wherein the handoff is performed without utilizing any signal-to-noise measures for pilot signals at the new frequency.
26. The method of claim 25 wherein the utilizing step includes utilizing the trigger metric to control a handoff to a particular pilot selected from a list of neighbor pilots stored in a database for a corresponding cell of the system.
27. The method of claim 25 wherein the measure of mobile receive power is used to determine when the mobile station makes reports of pilot signal strength.
28. An apparatus for use in controlling frequency handoffs in a wireless communication system in which a mobile station communicates with one or more base stations, the apparatus comprising:
a processor for generating a trigger metric as a measure of receive power at the mobile station; and a memory for at least temporarily storing a representation of the trigger metric, such that the trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call in the system, wherein the handoff is performed without utilizing any signal-to-noise measures for pilot signals at the new frequency.
a processor for generating a trigger metric as a measure of receive power at the mobile station; and a memory for at least temporarily storing a representation of the trigger metric, such that the trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call in the system, wherein the handoff is performed without utilizing any signal-to-noise measures for pilot signals at the new frequency.
29. The apparatus of claim 28 wherein the trigger metric is used to control a handoff to a particular pilot selected from a list of neighbor pilots stored in a database for a corresponding cell of the system.
30. The apparatus of claim 28 wherein the processor utilizes the measure of mobile receive power is used to determine when the mobile station makes reports of pilot signal strength.
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